A typical planetary transmission has a sun gear, a ring gear and a planetary carrier with a plurality of planetary gears rotatably mounted thereon, with the planetary gears engaging the ring gear and the sun gear. It is known in the art to connect to input and output of the planetary to varying combinations of the sun gear, the planetary carrier, or the ring gear, depending upon the precise transmission characteristics desired. The typical planetary is shifted between different gear ratios by using a clutch to connect pairs of the sun gear, planetary carrier, or ring gear against relative rotation, and/or by using a brake to fix one of the sun gear, planetary carrier or ring gear against rotation relative to the transmission housing.
When both a clutch and a brake are used, the most common control mechanism is a spring-biased hydraulic piston to control the brake and a separate spring-biased hydraulic piston to control the clutch. Depending upon the exact characteristics desired, the springs for the pistons can either bias the corresponding components into engagement or out of engagement, with hydraulic fluid then applied to the piston to disengage or engage the component, as desired.
In many situations, the brake and clutch should never be engaged simultaneously. This means that the timing of application of hydraulic fluid to the pistons for the two control mechanisms must be carefully controlled. This poses the problem that the control valve structure for these fluids must be very carefully designed and manufactured to ensure that this timing is correct.